The strength of ceramics is generally influenced by porosity, crystal grain size and the surface conditions thereof. The strength of sintered silicon nitride which has attracted attention as construction ceramics is also governed by these factors. Various sintering aids and sintering methods have been developed in an attempt to improve the strength of sintered silicon nitride. For example, Am. Ceram. Soc. Bull., Vol. 52, p. 560 (1973) reports that flexural strength reaches to 100 kg/mm.sup.2 in hot-press sintering, and Yogyo Kyokai Nenkai Koen Yokoshu (Transaction of Annual Meeting of Ceramic Society), Vol. 178 (1981) reports that flexural strength reaches 100 kg/mm.sup.2 in normal pressure sintering. In both reports, an improvement on strength is achieved by minimizing porosity.
Processes for producing an Si.sub.3 N.sub.4 --Y.sub.2 O.sub.3 --Al.sub.2 O.sub.3 sintered body using Y.sub.2 O.sub.3 as a sintering aid are described in JP-B-49-21091 and JP-B-48-38448 (the term "JP-B" as used herein means an "examined published Japanese patent application").
It is considered, as stated in the literature, that Si.sub.3 N.sub.4 having a .beta.-crystal lattice has a fibrous structure dispersed in a matrix phase to thereby improve strength and toughness. That is, the fact that the .beta.-crystal lattice of Si.sub.3 N.sub.4 is of a hexagonal system and isotropically grows in the direction of the c axis is positively taken advantage to improve strength. In some cases, the fibrous crystal grains grow in the longitudinal direction to more than 10 .mu.m as described in JP-B-48-38448 and Yogyo Kyokaishi (Journal of Ceramic Society), Vol. 94, p. 96 (1986).
However, since the strengthening mechanism in the above-described processes resides in growth of the fibrous structure, there is the possibility that abnormal grain growth or pore formation may accompany the fibrous structure growth. It is not believed, therefore, that the mechanism is always effective to improve strength.
In particular, in order to obtain a fibrous structure as shown in JP-B-49-21091, hot-press sintering should sometimes be employed, or it is sometimes necessary to add a .beta.-crystal Si.sub.3 N.sub.4 material which has previously been grown in the fibrous form by a heat treatment and to utilize the growth of this fibrous structure as described in Yogyo Kyokaishi (Journal of Ceramic Society), Vol. 94, p. 167 (1986).
Further, according to the processes of JP-B-49-21091 and JP-B-48-38448, Si.sub.3 N.sub.4 powder is ground and mixed together with oxide powder, e.g., Y.sub.2 O.sub.3 and Al.sub.2 O.sub.3, as a chief sintering aid and then sinter-formed, followed by prescribed processing to obtain a sintered body.
However, since the aid employed, e.g., Y.sub.2 O.sub.3 and Al.sub.2 O.sub.3, is not only hard but highly agglomerative due to the fineness of the primary particles thereof, the use of known grinding media limits mechanical grinding and mixing performance and one encounters difficulty in obtaining submicroscopic particles. Uniform mixing of the sintering aid is also limited.
Further, contamination by impurities arising from the grinding medium used cannot be avoided. The impurities remain in the sintered body to cause defects or non-uniformity of the sintered structure, resulting in a reduction of strength.